title: Sphingosine kinase 1 overexpression is regulated by signaling through PI3K, AKT2, and mTOR in imatinib-resistant chronic myeloid leukemia cells.
abstract: OBJECTIVE
  As a better understanding of the molecular basis of carcinogenesis has emerged, oncogene-specific cell-signaling pathways have been successfully targeted to treat human malignances. Despite impressive advances in oncogene-directed therapeutics, genetic instability in cancer cells often manifest acquired resistance. This is particularly noted in the use of tyrosine kinase inhibitors therapies and not more evident than for chronic myeloid leukemia. Therefore, it is of great importance to understand the molecular mechanisms affecting cancer cell sensitivity and resistance to tyrosine kinase inhibitors.
  
  
  METHODS
  In this study, we used continuous exposure to stepwise increasing concentrations of imatinib (0.6-1 μM) to select imatinib-resistant K562 cells.
  
  
  RESULTS
  Expression of BCR-ABL increased both at RNA and protein levels in imatinib-resistant cell lines. Furthermore, expression levels of sphingosine kinase 1 (SphK1) were increased significantly in resistant cells, channeling sphingoid bases to the SphK1 pathway and activating sphingosine-1-phosphate-dependent tyrosine phosphorylation pathways that include the adaptor protein Crk. The partial inhibition of SphK1 activity by N,N-dimethylsphingosine or expression by small interfering RNA increased sensitivity to imatinib-induced apoptosis in resistant cells and returned BCR-ABL to baseline levels. To determine the resistance mechanism-induced SphK1 upregulation, we used pharmacological inhibitors of the phosphoinositide 3-kinase/AKT/mammalian target of rapamycin signaling pathway and observed robust downmodulation of SphK1 expression and activity when AKT2, but not AKT1 or AKT3, was suppressed.
  
  
  CONCLUSIONS
  These results demonstrate that SphK1 is upregulated in imatinib-resistant K562 cells by a pathway contingent on a phosphoinositide 3-kinase/AKT2/mammalian target of rapamycin signaling pathway. We propose that SphK1 plays an important role in development of acquired resistance to imatinib in chronic myeloid leukemia cell lines.
authors.affiliation: Department of Experimental Medicine and Biochemical Sciences, University of Rome Tor Vergata, Rome, Italy. gabriellamarfe@libero.it
authors.forename: Gabriella
authors.initials: G
authors.lastname: Marfe
authors.forename: Carla
authors.initials: C
authors.lastname: Di Stefano
authors.forename: Alessandra
authors.initials: A
authors.lastname: Gambacurta
authors.forename: Tiziana
authors.initials: T
authors.lastname: Ottone
authors.forename: Valentina
authors.initials: V
authors.lastname: Martini
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Abruzzese
authors.forename: Luca
authors.initials: L
authors.lastname: Mologni
authors.forename: Paola
authors.initials: P
authors.lastname: Sinibaldi-Salimei
authors.forename: Paolo
authors.initials: P
authors.lastname: de Fabritis
authors.forename: Carlo
authors.initials: C
authors.lastname: Gambacorti-Passerini
authors.forename: Sergio
authors.initials: S
authors.lastname: Amadori
authors.forename: Raymond B
authors.initials: RB
authors.lastname: Birge
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis; D001483:Base Sequence; D001549:Benzamides; D015152:Blotting, Northern; D015153:Blotting, Western; D017931:DNA Primers; D019008:Drug Resistance, Neoplasm; D006801:Humans; D000068877:Imatinib Mesylate; D020014:K562 Cells; D015464:Leukemia, Myelogenous, Chronic, BCR-ABL Positive / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D010879:Piperazines / Q000494:pharmacology*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D011743:Pyrimidines / Q000494:pharmacology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D015854:Up-Regulation; D000099157:Sphingosine Kinase
chemical_list: D000970:Antineoplastic Agents; D001549:Benzamides; D017931:DNA Primers; D000068877:Imatinib Mesylate; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D010879:Piperazines; D051057:Proto-Oncogene Proteins c-akt; D011743:Pyrimidines; D058570:TOR Serine-Threonine Kinases; D000099157:Sphingosine Kinase; C546842:MTOR protein, human